1. Field of the Invention
The present invention relates to a reflection-type photo mask principally for use in X-ray projection exposure during the manufacturing process of semiconductor devices and the like.
2. Description of the Prior Art
Attention has been focused recently on lithography utilizing the X ray of a shorter wave length compared with ultraviolet ray. X-ray lithography is considered advantageous in that it can achieve higher resolution. This high resolution is theoretically impossible for lithography utilizing ultraviolet rays.
According to conventional X-ray lithography, so-called transmission-type X-ray masks have been used in which a desirable pattern is formed with a thin film member comprising an X-ray absorbing material onto a membrane of a thickness of about 2 .mu.m and comprising a material with relatively good X-ray transmission, for example silicone nitride. With X-ray irradiation these masks, a projection image of the mask pattern by the X ray transmitting through the part except for the pattern part is generated on the photo resist layer on the surface of wafer, which is employed for effecting lithography.
The following problems have been suggested about such transmission-type X-ray masks because thin membranes are employed in such masks.
1. Because the membranes per se are thin, such masks are readily breakable, and handling is difficult. PA0 2. Because the membranes are in the form of a thin film, deformation is induced in the membranes due to the inner stress in X-ray absorbing materials, with resulting occurrences of positional distortion in the projected pattern image. PA0 3. On X-ray irradiation, the temperature of the masks is raised due to the X ray energy absorbed into the membranes per se and the X-ray absorbing materials, and thermal expansion of the membrane and x-ray absorbing matter develops positional distortion in the pattern. PA0 (a) a process of forming an X-ray reflectable multilayer on a substrate formed from a material; PA0 (b) a process of forming a desirable resist pattern on the multilayer; PA0 (c) a process of irradiating an ion beam onto the multilayer on which is formed the resist pattern, from a preliminarily defined slant direction, to effect etching of the multilayer in the form of the resist pattern; and PA0 (d) a process of removing the resist pattern after the completion of the etching process.
In order to solve these problems, reflection-type X-ray masks have been developed. In reflection-type X-ray masks, substrates in the form of thick board are used, instead of thin membranes. FIG. 2a shows one example of such masks such that an X-ray reflectable multilayer 101 is formed on the entire surface thereof and a pattern is formed with X-ray absorbing material 105. FIG. 2b shows that on X-ray unreflectable substrate 102 a desirable pattern is formed with X-ray reflectable multilayer 101.
By means of an optical system, the light transmitted through a mask generates an image on the surface of a substrate in transmission-type X-ray masks, while the light reflected on the pattern surface of a mask generates an image on a substrate in such reflection-type X-ray masks. Because a thin membrane is not utilized in such reflection-type masks, the individual problems described above can be solved which are encountered with transmission-type masks.
However, a newly developed problem concerning such reflection-type X-ray masks has been suggested. When using a reflection-type X-ray mask 106, as is shown in FIG. 3, it is required that the optical axis of the incident light 103 be arranged not to be co-incident with the optical axis of reflection light 104, in order that optical system 107 which works to generate an image on substrate 108 from the reflected light 104 on the mask 106 should not interfere with the incident light 103 into the mask 106. Therefore, the incidence of the X ray into the mask 106 is absolutely required to be arranged in the form of grazing incidence, not in the form of vertical incidence.
The X ray incident into multilayer 101 (FIGS. 2a and 2b) of a mask penetrates into the multilayer in the depth direction thereof at some degree, and thereafter the X ray is partially on the interface of the individual layers within the multilayer. Hence, the partially overlaid reflection on a large number of the individual layers within the multilayer is utilized as the reflected X ray for generating an image.
For this reason, the intensity of reflected X ray 104 is lowered near the edge on the opposite side to the direction of X-ray incident direction of a pattern 105 of an absorbing material, where the pattern of the absorbing material 105 is formed on the X-ray reflectable multilayer (see FIG. 2a). FIG. 4a, concerns the reflected X ray which exits from the multilayer exposing part near the edge. The part of the X ray should be incident to multilayer and should essentially form reflected X ray disturbed by the pattern 104 of an absorbing material. Consequently, no overlap is observed in some of the reflected X ray 104 on the individual interfaces at the portion of the pattern. This produces consequent a lowered reflection intensity. Therefore, the distribution of intensity I of reflected X ray, on position P in the pitch direction of the mask pattern, has a diversity in the form of exponential curve. In FIG. 4a W represents the width of a pattern.
When a pattern comprising X-ray reflectable multilayer 101 is formed on an X-ray unreflectable substrate 102 (see FIG. 2b), a problem is suggested about the reflection intensity in the proximity of both edges on the incident and exit sides to the incident direction of X ray, as is shown in FIG. 4b.
There is less overlapping of reflected X ray on the interfaces of individual layers, because incident X ray 103 does not reach a sufficient depth within the multilayer 101 so that no reflection is induced from the reflection surface below. Therefore, the intensity of the reflected X ray 104 on the X-ray incident side has a distribution in the form of exponential curve as is shown in FIG. 4b. Thus, the reflection intensity on the pattern edge is low.
The X-ray which is reflected within the multilayers 101 and exits from the edge part constitutes the pattern. The intensity of the X ray also has a distribution in the form of exponential curve, as is shown in FIG. 4b.
With respect to FIG. 4b, it may be considered that the influences on both of the edges of the pattern work to negate each other. As is shown in the figure, however, a problem occurs in that the width Wr of the range in which the quantity of light I.sub.R to be required for exposure of resist generally is changed into a different one from the original pattern width W of a mask.
The diversity in the form of exponential curve of the distribution of the reflection intensity on (and around) these edges is represented in either case approximately as exp(-.mu.X/sin.theta.) if X, .mu. and .theta. are defined as representing the position in coordinate in the same direction as P, a linear coefficient and incident angle, respectively.
When the X ray of a wave length of 124 angstroms irradiates a multilayer comprising Mo/Si at an angle of 45.degree., for example, the position with a 1/e-fold reflection intensity corresponds to a position in distance by about 0.13 .mu.m apart from the pattern edge, which is never negligible in a projection image of a microfine circuit pattern.
As has been described above, a serious problem has been suggested about conventional reflection type mask for X-ray exposure in that a pattern on a mask cannot be precisely transferred on a substrate in either type as in FIG. 2a (FIG. 4a) or FIG. 2b (FIG. 4b).